Musannaf Ibn Abi Shayba 7:488

Abu Mu'awiyah narrated to us, from al-A'mash, from 'Amr ibn Murrah, from Salim ibn Abi al-Ja'd, from Shurahbil ibn al-Simt, who said: We said to Ka'b ibn Murrah, "Narrate to us from the Messenger of Allah (may Allah bless him and grant him peace), and be cautious." He said: I heard the Messenger of Allah (may Allah bless him and grant him peace) saying: "Whoever frees a Muslim man, it will be his ransom from the Fire, every bone of his [the freed man] replacing a bone of his [the emancipator]. And whoever frees two Muslim women, they will be his ransom from the Fire, every bone of his [the emancipator] being replaced by a bone of theirs."